More Clarity on IVC POCUS for Fluid Responsiveness

Written by Clay Smith

Spoon Feed
In intubated patients, respiratory variation of IVC diameter by 16% had fairly high diagnostic accuracy for predicting fluid responsiveness, but only when the tidal volume (TV) was ≥8 mL/kg and positive end-expiratory pressure (PEEP) ≤5 cm H2O.

Why does this matter?
Variation in IVC diameter on ultrasound was thought to be a great way to non-invasively assess fluid responsiveness.  Subsequent studies cast doubt on the accuracy of this method.  So how does it perform in a pooled analysis of mechanically ventilated patients?

Variation is variable
This was a meta-analysis of 12 studies with 753 mechanically ventilated patients.  Most studies were relatively small but had low risk of bias.  The gold standard used to measure fluid responsiveness in most studies was echocardiography.  Combining all studies, sensitivity was 73% and specificity was 82%.  The best ΔIVC threshold for predicting fluid responsiveness was 16%.

In subgroup analysis, they found improved diagnostic accuracy, "with TV ≥8 mL/kg and PEEP ≤5 cm H2O than in the group ventilated with TV <8 mL/kg or PEEP >5 cm H2O."

  • Sensitivity (0.80 vs 0.66; P = .02)
  • Specificity (0.94 vs 0.68; P < .001)
  • Odds ratio (68 vs 4; P < .001)
  • Area under the curve (0.88 vs. 0.70; P < .001)

This finding makes sense. In a mechanically ventilated patient, higher PEEP leads to higher intrathoracic pressure at the end of expiration (when the IVC is the flattest). This higher ITP causes the IVC to be more dilated, which leads to decreased IVC respiratory variation. Conversely, lower tidal volume decreases intrathoracic pressure at the end of inspiration (when the IVC is the largest). This causes the IVC to be less dilated, which leads to decreased IVC respiratory variation. In both cases, ultrasound will lead to false negatives for detecting fluid responsiveness. 

Admittedly, this is a hotly debated topic.  See the point/counterpoint posts below.

Source
Does Respiratory Variation in Inferior Vena Cava Diameter Predict Fluid Responsiveness in Mechanically Ventilated Patients? A Systematic Review and Meta-analysis.  Anesth Analg. 2018 May 21. doi: 10.1213/ANE.0000000000003459. [Epub ahead of print]

Another Spoonful

Peer reviewed by Thomas Davis

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